Four-wire transmission system



G. A. CAMPBELL.

FOU R-WIRE TRANSMISSION SYSTEM. APPLICATION FILED JAN. 11. 1916.

1,352,786. S Patentedsepfi 14,1920.

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UNITED STATES PATENT OFFICE.

GEORGE A. CAMPBELL, OF MONTCLAIR, NEW JERSEY, ASSIGNOR TO AMERICAN TELE- PHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

FOUR-WIRE TRANSMISSION. SYSTEM.

- To all whom it may concern:

Be it known that I, GEORGE A. CAMPBELL,

residing at Montclair, in the county of Essex and State of New Jersey, have invented certain Improvements. in Fourire Transmission bystem, of which the following is a specification.

This invention relates -to a telephone transmission system and more particularly toa telephone transmission system in which repeaters or amplifiers are employed. One ob ect of the invention is to provide a more effective and more economical circuit and one in which with a given grade of conductor a higher amplification and consequently greater volume of received speech may be secured than is possible in arrangements known to the art without reaching the unstable condition known as singing. Another object of the invention is to provide a circuit in which a higher amplification may be secured without reaching a condition which impairs intelligibility. These objects of my invention are attained by providing two separate transmission lines, one of which transmitstelephonic waves in one direction only, and the other of which transmits in the opposite direction only. and in relating said lines to common terminal stations for two-way communication.

Other and further objects of the invention may be understood by reference to the following description when read in connection with the accompanying drawing which is a diagram illustrating the transmission system of my invention. Referring to the drawing 1 and 2 are separate transmission lines constituting a four-wire transmission system. Line 1 includes a plurality of oneway repeaters, 3, 4;, which amplify telephonic waves propagatedfrom left to right. Line 2 includes a plurality of repeaters 5, 6

which amplify telephonic waves from right to left. Line 1 therefore transmits in one direction only while line 2 transmits in the opposite direction only. For convenience mechanical repeaters are shown inthe draw- .ing but it will be understood that any type of telephonic repeater may bev employed. At station A lines 1 and 2 are coupled to line 7 while at station E lines 1 and 2 are coupled to line 8. Lines and 8 mayterminate in central offices or in subscribers stations. The arrangements whereby the four-wire system is coupled to the terminating lines 7 designed to have an impedance equal to that of line 7 while the impedance of artificial line 10 is designed to be equal to that of line '8. When these conditions are satisfied impulses entering line 7 from line 2 .do not affect line 1. Similarly impulses entering Specification of Letters Patent. Patented Se t 1 4, i920;

line 8 from line 1 do not affect line 2. In

practice, of course, a perfect balance between the terminating lines and these associated artificial lines impossible to obtain and maintain. As a consequence there is always a tendency for impulses originating in either of the terminating lines to be repeated around the circuit consisting of I lines 1 and 2. W hen the unbalance reaches a certain value, said value being conditioned by the amplifying power oftherepeaters, a condition known to the art as singing is established. This tendency to sing is inherent in all repeater systems and seriously limits the amount of amplification possible.

.Thus, in the ordinary twoway two-repeater systems, the ratio of the repeated or amplified current in one line section, to the originating current in the other line section, is limited by the inherent and unavoidable imperfection of balance, and an excess of amplification sets up singing in the local repeater circuit. In the four-wire system of my invention, however, the amount of amplification possible increases with the length of the fourqvire line and the characteristic attenuation thereof, since the tendency to sing over a circuit including the transmission lines 1 and 2 in series, due to an increasein amplification, is neutralized by the corresponding increase in the total attenuation of said lines as the length of the lines is increased. Therefore, the conditions of balance remaining the same, so long as the total attenuation of the pair of lines exceeds the increase of amplification of the re peaters, the system cannot sing. For instance, if stations A and B be brought together so that lines 1 and 2 are quite short, as is the case in the ordinary two-way tworepeater system, it is possible toadjust the amplifiers so that the current leaving the system over outgoing line 8 is equal to, or somewhat greater than the current entering the system from line 7 without any resultant singing. If now, stations A and B be no I ' the conditions of balance remaining the for theduplication of line required by the same, the am lification maybe increased un-' til'substantia ly the same currentleaves the system over line 8 as before, and no singing will result, since theincrease in amplification tending to cause singing is balancedby a corresponding increase in attenuation due to the increased length oi lines 1 and 2. It

'will thus be seen that however widely stations A and B may be separated geographically, the ma be brought together .electr'i cally,"an in act, the amplification can be adjusted so as to secure a gain in tra'nsmis-- sion at station B as compared with station A, without any singing taking place.

In other words the system is so arranged that the amplification of the one way re:

. lines by an amount "greater than the. loss due to the meansl'for balancing the terminat-- peaters in the two'transmission lines coIn prising the four-wire circuit shall not exceed the loss due to the attenuation of said ing arrangements.- With the system thus adjusted, a w'aye impressedupon oneof the lines and transmitted over a, closed circuit including said transmission lines and interconnecting means, will arrive at-the originating point decreased in amplitude, 'so'that sin g will not take place.

he advantages att'nding fourwire system of transmission, disclosed above, are

man and important. Inordinary transmission. systems employing repeaters, the amount of amplification possible is strictly limitednsheretofore stated, by the fact that the balancing network can only be at best imperfect and that therefore there i more or less interaction between t and output side of a repeater, said inter:

s always he input action iving'rise to the condition of susvtained nteraction-known fassinging. Asi a consequence the amplification is limited by the degree of said unbalance. Inthe four-wire system very high amplification'is possible without singing; in facttheoretically with a. single repeater an amplificationis possible without singin' which entirely neutralizes the current dnninution' con'se;

sent.

quent upon the line attenuation, irrespective of the length of the line. Further if'frepeaters are operated in tandem-on an ordinary two-wire system interaction between repeaters limits the amplification of each repeater and necessitatesa definite separa Jtion between repeater stations. ln-the fourwire s stem this limitation is entirely abn the four-wire system also balancingnetworks are required only at the two terminalsof the four-wire lines whereas in the two-wire system balancing networks or their fequivalents are ired at each repeater station. In many systems wherein high lguality and large volumeof received -s $1 0.1 is des11'ed,- the above-mentmifed a vantages more than compensate four-wire transmission system. four wire systems it is possible because of the advantages above' pointed out, to use low grade conductors for the two transmission lines of the four wire syst em, so that In other {the cost of such conductors, together with that of amplifiers worked at high amplification, is less than the cost of a two Wire sysf tem using high grade conductors with repeaters worked at'low amplification.

While the invention has been illustrated as mission lines, the total transmission loss.

over the circuit comprisin said transmisslon llnes and interconnecting means-b'emg at least equal to the amplification due to the repeaters. I

2. A telephone transmission system comprising two stations,two separate transmission lines connecting said stations; a one: way repeater in one of-said transmission lines, said repeater amplifying and; trans-,

, mitting' impulses in one direction only; a

one-way repeater in the other of said transmission lines amplifying and transmitting impulses in the opposite direction only, and

means at said stations interconnecting the ends of said lines, said means being so arranged that the transmission from one line to the other is relatively small, the total transmission loss over the circuit comprisampli' cation due to the repeaters,

'31'A four ire transmission system comprising two-stations, terminating lines at said stations, two separate transmission lines connecting said terminating lines, one-way repeaters inone of said transmission lines g said transmission lines and intercom nectin means, beingat least equal to the 1 repeating in one direction, one-way repeaters in the other of sand transmisslonllnes' lines, said ing said transmission lines and interconnectin means being at least equal to the ampll cation due to'the repeaters.-

4. A telephone'transmisslon system comprising two separate transmission lines, one of said transmission lines transmitting telephonic impulses in one direction only, and the other of said transmission lines transmitting telephonic impulses in the opposite direction only; a plurality of one-way repeatersfin each of said transmission lines, and means, interconnecting the ends of said lines, said means being so arranged that the transmission from one line to the other is relatively small, the total transmission loss over the circuit comprising said transmission lines and interconnecting means being at least equal to the amplification due to the repeaters. v

5. A transmission system comprising two stations each having an input circuit and an output circuit, two transmission lines connecting said stations, the input circuit of one of said stations being connected to one of said transmission lines and its output circuit being connected to the other of said transmission lines, the input circuit of the other station being connected to the second of said transmission lines and its output'circuit being connected to the first of said transmission lines whereby one of said transmission lines transmits in one direction and the other of said transmission lines transmits in the opposite direction, a one-wayrepeater in each of said transmission lines, and means interconnecting the input and output circuit of each station, said means being so arranged that the transmission from one line to the other is relatively small, the total transmission loss including the said transmission lines, input and output circuits, and interconnecting means, being at least equal to the amplifica-. tion due to therepeaters.

6. In a telephone system, the combination of two-separate transmission lines constituting a four-wire transmission system extending between two stations, terminating lines at eachof said stations associated with the ends of said four-wire transmission system, artificial lines at said station for balancing each of said terminatingylines, and amplifying means in each of said transmission lines so adjusted that the amplification of said amplifying means shall not exceed the attenuation of said transmission lines by an amount greater than the loss due to saidbalancing means.

7, In a telephone system, two stations, a four-wire transmission circuit extending between said stationscomprising two transmission paths, terminating lines at each station, means for associating said terminating lines with said four-wire circuit, said means being so arranged that the transmission from one path to the other is relatively small, and means to amplify the current transmitted from said connecting lines to said founwire circuit, the total transmission loss over the four-wire ClIClllt: and sand 1nductlve assoclating means belng" at least equal to the amplification due to the amplifying means.

8. A transmission system comprising two stations, two transmission lines connecting said stations for transmission in opposite directions, terminating arrangements at said stations to which the ends of said lines are connected, balancing means for said terminating arrangements and amplifiers in said lines so adjusted that the amplification of said amplifiers shall not exceed the attenuation of said lines by an amount greater than the loss due to said balancing means.

9. A transmission system comprising two due to the means for associating the ends of said transmission lines with the terminating lines and balancing means.

10. A transmission'system comprising two stations, two transmission lines connecting said stations for transmission in opposite directions, means at said stations interconnecting the ends of said lines, said means being so arranged that the transmission from one line to the other is relatively small, and one-way repeaters in said lines, the amplification of the repeaters and the attenuation of said lines being such that a'wave impressed upon, one of said lines and transmitted over a closed circuit, including said transmission lines and interconnecting means, will arrive at the originating point decreased in amplitude.

11. A transmission system comprising two stations, two transmission lines connecting said stations fortransmission in opposite directions, terminating arrangements forsaid transmission lines, will arrive at the originating point decreased inamplitude.

12. A transmission system comprising two stations, terminating lines at said stations,

lines by an amount greater than the loss 7 balancing means forthe terminating lines, I

two transmission lines connecting said Sta-- t1ons for transmission *in opposite direc- J I i H tion s,omeans at,said statio ns interconnecting the ends of said transmission lines with said terminating lines and balancing means and .one-way repeaters in said transmission lines, the amplification of the repeaters and the attenuation of said transmission lines being such that a Wav impressed upon one of said transmission lines and transmitted over a closed circuit, including said trans l0- mission lines and interconnecting means,

will arrive at the originating point decreased in amp'1itude.,j c =In testimony whereof, I have signed my hame to this specification in the presence of two subscribing witnesses, this thirteenth 15 day p f Januar ,1916.

/ G ORGrEr A. CAMPBELL.

Witnesses: I

FRED; S. RoBmsoN, RALPH WOLF. 

